The influence of intrinsic and extrinsic factors, such as antibiotic resistance, genetic characteristics, host immune response, on the incidence and pathogenicity of P. aeruginosa in cancer patients is the prime objective of this study. Preliminary data on antibiotic resistance transfer suggest that nonconjugative plasmids may be responsible for the resistance patterns in various strains of Pseudomonas spp. recovered from sink drains. In C3H adult female mice, IgM is the Ig class primarily responsible for protection during the first 2 weeks after immunization, whereas nonagglutinating IgG is responsible for later protection; both are opsonins. Importance of humoral response in the control of P. aeruginosa infection is an ongoing study. The kinetics of Ig class specific antibiody to P. aeruginosa in sera of cancer patients will be studied. Intraspecies differentiation of Pseudomonas spp. other than P. aeruginosa by antisera is under investigation.